How do I find out why Spamcop keeps flagging my dynamic IP addresses?

I keep having to go into the router and renew my IP address every couple of days because my IP addresses get flagged by Spamcop. My email service provider uses Spamcop's blacklist. So, outgoing emails get blocked when my IP is blacklisted. We have 6 computers at my business. I've checked all of them for anything suspicious, ran malware scans on each one, and nothing. We do have a piece of software that sends automatically sends me notifications by email, but that only happens about 3 times a day. We have another business with an almost identical setup and that shop has no Spamcop issues (same email provider). All I can think of is one of these computers is sending out enough SMTP traffic for Spamcop to think we're spamming. How can I find out which computer? Is there some type of software that can do this?